This would be almost same like Arenas, except for rounds and groups, it would be 1 Big boss-like monster each round. Each requiring a special tactic to kill them.
The should be 3 or 5 rounds, after each round u can choose if u continue or quit. Of course, the further u get, the more rewarding it would become.
This should fit nicely in tier 3. Or the bosses could drop boss tokens also, which at a certain amount u can trade for gear.
Use Tortodomes! (and more things, of course)
Doing ACTUAL bosses is a bit much. Seems like an easy spoiler. OOO wouldn't do that.
To make it a bit harder for people who know what they are doing, why not make it a 'double or nothing' scenario?
Stage one complete. You get the prize. Move on to stage two. Prize. But if at any point you give up, then you lose all the prizes you got.
BUT! In between each stage is an exit, like how they do in the regular arenas. The player is allowed to leave with his gained loot if he gets to that lift. If he chooses to go on, however, he will risk his loot for more loot. Lose, and he will lose it all.
Lose doesn't mean die btw. It's like pressing the 'return to haven' button.
Your idea is similar to one I've seen for the Advanced Training Hall, where players can train against various monster types. Perhaps we could modify your idea such that all the bosses in the game are put together in an arena-like fashion. There would be four stages, one right after the other: first is Snarbolax, second is Roarmulus Twins, third is Jelly King, and fourth is Vanaduke. Maybe the next level takes it up a notch, replacing these bosses with Shadow Lair bosses in the same stages, i.e., Rabid Snarbolax, Red Roamulus Twins, Ice Queen, and Darkfire Vanaduke. These could be either training levels or special, limited-time-offer levels where players can fight the bosses directly without having to go through the pre-levels.